This study documented mercury contamination in river otter (Lutra canadensis) in the Keweenaw Bay area, northern Michigan. Total mercury concentrations in liver and kidney from 11 otters were determined by atomic absorption spectroscopy and ranged from 0.79 to 4.07 μg g−1wet weight. There were no significant differences in mercury concentrations between males and females, or between tissue types. A significant difference did exist between mercury levels in mature animals and those in immature animals (<1 year old). Data indicate that otters in areas with no known point sources are contaminated with mercury.